Human monoclonal antibody (mAb) therapy offers considerable advantages for prophylaxis, preemptive and acute treatment in viral outbreak settings, yet its' use as a therapeutic agent against emerging viral pathogens has been protracted because of longstanding barriers such as antigenic variability of circulating viral strains and neutralization escape, particularly for RNA respiratory pathogens such as coronaviruses (CoVs) and influenza. Importantly, cost of manufacturing, a historically perceived limitation of mAb-based immunotherapy has dramatically decreased in recent years. Here, the Marasco, Baric and Liddington labs propose to develop platforms for rapidly identifying broad-spectrum neutralizing human mAbs (BnAbs) that are effective against emerging epidemic and zoonotic pool strains, using SARS-CoV as a model. Major strengths of this model include robust sequence database of epidemic and zoonotic isolates, synthetic genomics and reverse genetics and in vivo pathogenesis models in young and aged mice that recapitulate severe human end-stage lung disease and ARDS; atomic structures of SARS-CoV receptor ACE2 and human BnAbs bound to its Spike protein; and well developed methodologies and novel reagents developed by our collaborative team. Our approach is based on our published data that BnAbs can be designed to either prevent virus neutralization escape and/or attenuate virus evolution. Information gained from in vitro studies will be used to develop, in parallel, new strategies for maximizing vaccine efficacy against the broader heterogeneous pool of animal strains. The impact of these studies will be high, providing a template for similar design platforms for other important human emerging pathogens. Specifically, we will further build on our discovery of the potent human anti-SARS mAb 80R that is directed to the receptor binding motif (RBM) on Spike protein, and BnAb derivatives of 80R that have been subsequently engineered to be active against all SARS-CoV isolates from humans and animals (except bats). It is now known that the natural reservoir of many human pathogenic CoVs is the bat, where ancestral CoVs, including the bat-SARS-CoV, can replicate, recombine and reemerge as new human pathogens. The bat reservoir of SARS-CoVs is firmly established, and from the public health standpoint, emerging and re-emerging SARS-CoVs represent a real threat to human health. We propose to simultaneously develop BnAbs against the conserved S2 domain of SARS-CoVs, which may have even greater breadth (including activity against new SARS-like-CoVs and other pathogenic CoVs that might emerge from bats in the future), potency of neutralization and resistance to neutralization escape. In 4 specific aims, we will test 12 hypotheses that relate to defining new nAbs against SARS-CoVs, their neutralizing epitopes, escape mechanisms and strategies to prevent neutralization escape. Co-crystal structures of BnAbs-Spike complexes solved during these studies will guide our virologic studies and antibody engineering efforts. The information gained from in vitro neutralization escape studies will also be used to design prototypic RBM- and S2-directed vaccine studies to determine if we can control the type of nAbs that are elicited, their breadth of neutralization and their resistance to neutralization escape, with the goal of eliciting BnAb responses that can block virus evolution.